1. Field of the Invention
This invention relates to a hot isostatic pressing apparatus (hereinafter abbreviated as "HIP apparatus" for brevity), and more particularly to an HIP apparatus equipped with a novel heating unit which permits a more effective use of the interior space of a pressure vessel and enlarges the height of the uniform temperature zone by 1.5-3 times that of a conventional heating unit equipped with a heater of the same height.
2. Description of the Prior Art
An HIP apparatus is one of many apparatuses which have recently found wide-spread commercial utility in a broad field led by the forming and sintering of powder and including the diffusion bonding and elimination of casting defects and have thus drawn the attention of engineers. It is generally operated under high-temperature and high-pressure conditions of at least 1,000.degree. C. and at least 1,000 kg/cm.sup.2. As a facet of recent trend, it has been attempted to operate such an HIP apparatus, especially, at high pressures reaching 2,000 kg/cm.sup.2 or so. In such a high pressure atmosphere, the heat capacity of a gaseous substance is increased to a considerable extent and the temperature distribution of the gaseous substance in a high-temperature and high-pressure furnace becomes significantly uneven. The length of the uniform temperature zone in the high-temperature and high-pressure furnace is rendered very short, thereby limiting the actually-effective length to only a part of the furnace. This is extremely uneconomical from the viewpoint of making efficient use of the internal spacing of the expensive high pressure vessel.
Taking a hint from the fact that a multistage and self-standing heater can be readily obtained by fixedly connecting a plurality of heating elements to one another with ring-shaped porcelain insulators, each multistage and self-standing heaters have generally been used in order to enlarge the constant temperature zones.
However, such conventional multistage and self-standing heaters are accompanied by a drawback in that they have become larger and the HIP apparatuses have hence been scaled up. Moreover, such are accompanied by another drawback in that the thermal stresses, which are developed due to difference in thermal expansion between the porcelain insulators and heaters, have become greater and parts having weaker strength are broken. They are also accompanied by a further drawback in that the selection of porcelain insulators having good electrical insulation and impact resistance at high temperatures leads to an increase in production cost and difficulties in production technique.
Coupled with actual utilization of an HIP apparatus, there is an increased recent demand for the development of a compact apparatus structure recently. There is thus an increasing demand for a heating unit having a simple structure and good heat uniformity.
The above-described trend has promoted the adoption of heating units which are each of the cylindrical and self-standing type having no porcelain insulators and individually have a simple structure making use of a single-stage heater. At the same time, it has become the primary concern to improve the uniformity in temperature in such a heating unit.